Compressed-air compressor

ABSTRACT

A compressed-air compressor, the compressor having a cylinder; a piston mounted to slide along the cylinder and define, inside the cylinder, a variable-volume compression chamber; and an actuator assembly connected to the piston to impart to the piston a reciprocating travel, along the cylinder, between a top dead centre position and a bottom dead centre position; the actuator assembly having a belt drive; and the cylinder having a vent hole formed, at most, at  60 % of the travel of the piston, measured from the top dead centre position.

The present invention relates to a compressed-air compressor.

More specifically, the present invention relates to a compressor of the type comprising a cylinder; a piston mounted to slide along the cylinder and define a variable-volume compression chamber inside the cylinder; and actuating means comprising a belt drive and connected to the piston to produce a reciprocating movement of the piston along the cylinder, between a top dead centre position and a bottom dead centre position.

BACKGROUND OF THE INVENTION

The belt drive normally comprises a belt mounted at one end to a first pulley fitted to the output shaft of a normally electric motor, and at the other end to a second pulley defining the crank of a connecting rod-crank mechanism for converting rotation of the first pulley into a reciprocating movement of the piston along the cylinder, between the bottom dead centre position (end of the intake stroke) and top dead centre position (end of the compression stroke).

When the motor in compressors of the above type is stopped, the residual compressed air inside the cylinder and the way in which the crank mechanism is normally mounted tend to move the piston towards the bottom dead centre position, with the result that the first stroke of the compressor, when the electric motor is started up again, is normally a compression stroke.

Because of this and the fact that the electric motors normally employed have a relatively high breakaway torque and acceleration, the reaction forces increase rapidly when the motor is started up again, and may result in snapping of the drive belt.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compressor of the above type, designed to eliminate the above drawback.

According to the present invention, there is provided a compressed-air compressor as claimed in Claim 1 and preferably in any one of the following Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of non-limiting embodiments of the invention will be described by way of example with reference to the attached drawings, in which:

FIG. 1 shows an axial section of a preferred embodiment of the compressor according to the present invention;

FIG. 2 shows an operating graph of the FIG. 1 compressor.

DETAILED DESCRIPTION OF THE INVENTION

Number 1 in FIG. 1 indicates as a whole a compressor comprising a vertical plate or frame 2, which has a through hole 3 with a horizontal axis 4 and engaged in rotary manner by the output shaft 5 of a motor 6—in the example shown, an electric motor—fitted to frame 2 and for driving a pulley 7 fitted to an end portion of shaft 5 and located on the opposite side of frame 2 to motor 6.

Frame 2 has a further through hole 8, which has an axis 9 parallel to axis 4 of hole 3 and defining, with axis 4, a vertical plane coincident with the FIG. 1 plane. Hole 8 is located a given distance from hole 3, and is engaged in rotary manner by a shaft 10 of a further pulley 11, which is coplanar with pulley 7, is connected to pulley 7 by an endless belt 12, and defines, with pulley 7 and belt 12, a belt drive 13 for transmitting rotation of shaft 5 to pulley 11.

The top end of frame 2 has a lateral appendix 14, which lies in a horizontal plane perpendicular to the plane defined by axes 4 and 9, and to the plane of frame 2, and has a through hole 15 with a vertical axis 16 perpendicular to axes 4 and 9 and lying in the plane defined by axes 4 and 9. A tubular body, supported by lateral appendix 14, is fitted through hole 15, coaxially with axis 16, and defines a cylinder 17 open at the bottom and closed at the top by a head 18, which has a chamber 19 with a hole 20 for drawing in outside air through a filter 21, and a chamber 22 separate from chamber 19 and having a compressed-air delivery hole 23.

A piston 24 is mounted to slide in fluidtight manner inside cylinder 17, has a rod 25 projecting downwards from cylinder 17, and defines, inside cylinder 17, a variable-volume chamber 26, which communicates alternately with chamber 19 and chamber 22 via a known blade valve assembly 27 interposed between the top of cylinder 17 and head 18.

Piston 24 is operated by an actuator assembly 28, which comprises motor 6, belt drive 13, and a connecting rod-crank mechanism 29 of the type known as “rod-less”, in which pulley 11 defines the crank, and rod 25 acts as a connecting rod integral with piston 24, which oscillates inside cylinder 17 as it moves along chamber 26. More specifically, pulley 11 is interposed between frame 2 and rod 25, and, on the side facing rod 25, has an eccentric pin 30, which has an axis 31 parallel to axes 4 and 9, and engages in rotary manner a hole 32 formed through an end 33 of rod 25. A cylindrical ventilator 35, integral with pulley 11 and coaxial with axis 9, is fitted by an axial screw 34 to the free end of pin 30 projecting from hole 32.

In actual use, rotation of shaft 5 of motor 6 produces rotation of pulley 11 about axis 9, and a reciprocating movement of piston 24, along cylinder 17, between a bottom dead centre position (FIG. 1), to which piston 24 returns naturally whenever motor 6 is stopped, and a top dead centre position (not shown), in which the volume of chamber 26 is substantially reduced to zero, and piston 24 is positioned substantially contacting valve assembly 27.

To prevent excessive rebound on belt 12 whenever motor 6 is started up, a radial vent hole 36 is formed through the lateral wall of cylinder 17 to permit no-load operation of piston 24 at least when motor 6 is started up.

To avoid seriously reducing flow during normal operation of compressor 1, it was first decided to form hole 36 just above piston 24 in the bottom dead centre position. This solution was ultimately ruled out, however, due to piston 24 not always stopping exactly in the bottom dead centre position, and even only a slight deviation of the piston from the bottom dead centre position resulting in closure of hole 36, thus making it ineffective.

Contrary to all expectations, it was discovered that:

-   -   forming hole 36 at 60% of the travel D of piston 24, measured         from the top dead centre position, substantially ensures the         piston will be positioned below hole 36 when motor 6 is stopped;     -   if hole 36 is formed at 40-60% of the travel D of piston 24,         measured from the top dead centre position, the reduction in         flow (compared to a cylinder 17 with no hole 36) is negligible;     -   the best result is achieved forming hole 36 at 53% of the travel         D of piston 24, measured from the top dead centre position.

This is shown clearly in the FIG. 2 graph, in which curve A shows the variation in flow of a compressor 1 with no hole 36, and curve B the variation in flow of a compressor 1 with a hole 36 formed at 53% of the travel D of piston 24, measured from the top dead centre position. As shown clearly in the FIG. 2 graph, for delivery pressures over 2 bars, the reduction in flow due to hole 36 is practically nil. 

1) A compressed-air compressor, the compressor (1) comprising a cylinder (17); a piston (24) mounted to slide along the cylinder (17) and define, inside the cylinder (17), a variable-volume compression chamber (26); and actuating means (28) connected to the piston (24) to impart to the piston (24) a reciprocating travel (D), along the cylinder (17), between a top dead centre position and a bottom dead centre position; the actuating means (28) comprising a belt drive (13); and the compressor (1) being characterized in that the cylinder (17) has a vent hole (36) formed, at most, at 60% of the travel (D) of the piston (24), measured from the top dead centre position. 2) A compressor as claimed in claim 1, wherein the vent hole (36) in the cylinder (17) is formed at 40-60% of the travel (D) of the piston (24), measured from the top dead centre position. 3) A compressor as claimed in claim 1, wherein the vent hole (36) in the cylinder (17) is formed at 53% of the travel (D) of the piston (24), measured from the top dead centre position. 4) A compressor as claimed in claim 1, wherein the actuating means (28) comprise a support (2); a motor (6) fitted to the support (2) and having an output shaft (5); and said belt drive (13), which comprises a first pulley (7) fitted to the output shaft (5), a second pulley (11) fitted in rotary manner to the support (2), and an endless belt (12) looped about the first and second pulley (7, 11). 5) A compressor as claimed in claim 4, wherein the actuating means (28) also comprise a connecting rod-crank mechanism (29) for converting rotation of the first pulley (7) to a reciprocating movement of the piston (24), along the cylinder (17), between the bottom and top dead centre positions; the second pulley (11) defining a crank of the connecting rod-crank mechanism (29). 